A field-emission cathode (or field emitter) contains a group of electron-emissive elements that emit electrons upon being subjected to an electric field of sufficient strength. The electron-emissive elements are typically situated over a patterned layer of emitter electrodes. In a gated field emitter, a patterned gate layer typically overlies the patterned emitter layer at the locations of the electron-emissive elements. Each electron-emissive element is exposed through an opening in the gate layer. When a suitable voltage is applied between a selected portion of the gate layer and a selected portion of the emitter layer, the gate layer extracts electrons from the electron-emissive elements at the intersection of the two selected portions.
The electron-emissive elements are often shaped as cones. Referring to the drawings, FIGS. 1a-1d illustrate a conventional technique as, for example, disclosed in Spindt et al, U.S. Pat. No. 5,559,389, for creating conical electron-emissive elements in a gated field emitter for a flat-panel CRT display. At the stage shown in FIG. 1a, the partially finished field emitter consists of substrate 20, emitter-electrode layer 22, dielectric layer 24, and gate layer 26. Gate openings 28 extend through gate layer 26. Corresponding dielectric openings 30 extend through dielectric layer 24.
Using a grazing-angle deposition procedure, lift-off layer 32 is formed on top of gate layer 26 as depicted in FIG. 1b. Emitter material is deposited on top of the structure and into dielectric openings 30 in such a way that the apertures through which the emitter material enters openings 30 progressively close. Generally conical electron-emissive elements 34A are thereby formed in composite openings 28/30. See FIG. 1c. Layer 34B of excess emitter material simultaneously forms on top of gate layer 26. Lift-off layer 32 is subsequently removed to lift off excess emitter-material layer 34B. FIG. 1d shows the resultant structure.
At the stage shown in FIG. 1c, excess emitter-material layer 34B provides a barrier between electron-emissive cones 34A and the external environment. The presence of the barrier provides an opportunity to perform additional processing on the partially finished field emitter while excess layer 34B prevents cones 34A from being contaminated by materials that come into contact with the field emitter during the additional processing. However, the benefit provided by the barrier is reduced if excess layer 34B is porous to any of these materials. Accordingly, it is desirable to inhibit such materials from passing through excess emitter material, such as layer 34B, and contaminating cones 34A.
Also, the use of lift-off layer 32 to remove excess emitter-material layer 34B can be cumbersome. For example, the deposition of lift-off layer 32 must be performed carefully to assure that no lift-off material accumulates on emitter-electrode layer 22 and causes electron-emissive cones 34A to be lifted off during the lift-off of excess layer 34B.
Wilshaw, PCT Patent Publication WO 96/06443, utilizes an electrochemical technique for removing excess molybdenum that accumulates over a gate layer during deposition of molybdenum through openings in the gate layer to form conical portions of the electron-emissive elements of a field emitter. No lift-off layer is employed in Wilshaw's electrochemical removal technique. Should it be beneficial to perform additional processing on a partially finished field-emitter while excess emitter material overlies electron-emissive elements, it is desirable that materials employed during the additional processing be inhibited from contaminating the electron-emissive elements regardless of whether the excess emitter material is removed by a lift-off or electrochemical technique.